Properties of the Two New Fluids in Minerals. 411 



those cavities where the vacuities are so large that the fluid its 

 converted into vapour before it fills them, the refractive power 

 of it, as measured when the cavities are full, will obviously be 

 much less than that which is obtained when the substance retains 

 its fluidity, and it will vary in different specimens, and even in dif- 

 ferent cavities of the same specimen, according to the proportions 

 which the vacuity bears to the quantity of the expansible fluid. 



Additional Observations on the New Fluids in Minerals. 



Having had occasion to shew the various phenomena of the 

 new fluids to several distinguished foreigners, and to others 

 who took an interest in the subject, I have thus been led to con- 

 tinue my examination of minerals in relation to these remark- 

 able substances. Some of my scientific friends, who were anxious 

 to repeat these experiments, have experienced great difficulty in 

 obtaining specimens of minerals containing the cavities of fluid. 

 This difficulty has no doubt arisen, from their examining the 

 well crystallised specimens which are generally found in the ca- 

 binets of mineralogists. If they had broken up with the ham- 

 mer only a few of the rounded or imperfectly crystallised white 

 topazes from Brazil or New Holland, they could scarcely have 

 failed to discover, with the compound microscope, innumerable 

 cavities fitted for the purposes of observation. After a little 

 practice in splitting and preparing the specimens, in which, from 

 the perfection of the cleavage planes, the aid of the lapidary is 

 almost never required, the patient observer will experience no 

 difficulty in detecting cavities of every variety of form, and in dis- 

 covering the fluid as it flows from the opened cavities over the 

 planes of cleavage. Mr SANDERSON, lapidary in Edinburgh, who 

 takes a great interest in every pursuit of a scientific nature, has 

 succeeded in obtaining some of the finest specimens of these new 

 fluids ; and by cutting and polishing the Topazes which contain 



